Implantable stimulation devices deliver electrical stimuli to nerves and tissues for the therapy of various biological disorders, such as pacemakers to treat cardiac arrhythmia, defibrillators to treat cardiac fibrillation, cochlear stimulators to treat deafness, retinal stimulators to treat blindness, muscle stimulators to produce coordinated limb movement, spinal cord stimulators to treat chronic pain, cortical and deep brain stimulators to treat motor and psychological disorders, and other neural stimulators to treat urinary incontinence, sleep apnea, shoulder sublaxation, etc. The description that follows will generally focus on the use of the invention within a Spinal Cord Stimulation (SCS) system, such as that disclosed in U.S. Pat. No. 6,516,227. However, the present invention may find applicability in any implantable medical device system.
As shown in FIG. 1, a SCS system typically includes an Implantable Pulse Generator (IPG) 100, which includes a biocompatible device case 30 formed of titanium for example. The case 30 typically holds the circuitry and battery 26 necessary for the IPG to function, although IPGs can also be powered via external RF energy and without a battery. The IPG 100 is coupled to electrodes 106 via one or more electrode leads (two such leads 102 and 104 are shown), such that the electrodes 106 form an electrode array 110. The electrodes 106 are carried on a flexible body 108, which also houses the individual signal wires 112 and 114 coupled to each electrode. In the illustrated embodiment, there are eight electrodes on lead 102, labeled E1-E8, and eight electrodes on lead 104, labeled E9-E16, although the number of leads and electrodes is application specific and therefore can vary. The leads 102 and 104 couple to the IPG 100 using lead connectors 38a and 38b, which are fixed in a header material 36, which can comprise an epoxy for example. In a SCS application, electrode leads 102 and 104 are typically implanted on the right and left side of the dura within the patient's spinal cord. These leads 102 and 104 are then tunneled through the patient's flesh to a distant location, such as the buttocks, where the IPG 100 is implanted.
FIG. 2A shows a plan view of an external controller 12 used to wirelessly communicate with the IPG 100, while FIG. 2B shows a cross section of the external controller 12 and the IPG 100. As shown in FIG. 2B, the IPG 100 typically includes an electronic substrate assembly 14 including a printed circuit board (PCB) 16, along with various electronic components 20, such as a microcontroller, integrated circuits, and capacitors mounted to the PCB 16. Two coils are generally present in the IPG 100: a telemetry coil 13 used to transmit/receive data to/from the external controller 12; and a charging coil 18 for charging or recharging the IPG's battery 26 using an external charger (not shown). The telemetry coil 13 can be mounted within the header 36 of the IPG 100 as shown, but can also be provided within the case 30, as disclosed in U.S. Patent Publication 2011/0112610 for example.
The external controller 12, such as a patient hand-held programmer or a clinician's programmer, is used to send data to and receive data from the IPG 100. For example, the external controller 12 can send programming data such as therapy settings to the IPG 100 to dictate the therapy the IPG 100 will provide to the patient. Also, the external controller 12 can act as a receiver of data from the IPG 100, such as various data reporting on the IPG's status. As shown in FIG. 2B, the external controller 12, like the IPG 100, also contains a PCB 70 on which electronic components 72 are placed to control operation of the external controller 12. The external controller 12 is powered by a battery 76, but could also be powered by plugging it into a wall outlet for example.
The external controller 12 typically comprises a graphical user interface 74 similar to that used for a portable computer, cell phone, or other hand held electronic device. The graphical user interface 74 typically comprises touchable buttons 80 and a display 82, which allows the patient or clinician to operate the external controller 12 to update the therapy the IPG 100 provides, and to review any relevant status information that has been reported from the IPG 100.
Wireless data transfer between the IPG 100 and the external controller 12 preferably takes place via inductive coupling between a telemetry coil 73 (FIG. 2B) in the external controller 12 and the telemetry coil 13 in the IPG 100. Either coil 13 or 73 can act as the transmitter or the receiver, thus allowing for two-way communication between the two devices. Typically, the transmitting device will send data to the receiving device via a Frequency Shift Keying (FSK) protocol in which different data states are indicated by different frequencies. For example, a transmitting device may send a logic ‘0’ bit to the receiving device at 121 kHz, but may send a logic ‘1’ bit at 129 kHz. That is, the data is represented relative to a center frequency fc=125 kHz, with the logic states representing a +/−4 kHz deviation from this center frequency. Bits may be serially transferred in this fashion at a given rate of 4 k bits/sec (4 kHz), i.e., a bit duration of tb=250 μs for example, meaning that a logic ‘0’ bit roughly comprises 30 cycles at 121 kHz (121/4), while a logic ‘1’ bit roughly comprises 32 cycles at 129 kHz (129/4). These frequencies are not significantly attenuated in the patient's tissue 25, and so data transmission can occur transcutaneously using this scheme.
FIG. 3 illustrates prior art receiver and demodulation circuitry 150 used in an external controller 12 to receive and recover FSK data transmitted from the IPG 100. The circuitry 150 includes a L-C tank circuit 151 (or antenna, more generally) comprising a serial connection between the telemetry coil 73 and a tank capacitor C. (A parallel arrangement can also be used). The inductance L of the coil 73 or the capacitance of the tank capacitor C can be tuned to generally allow the tank circuit 151 to resonate at the center frequency fc=125 kHz of the data expected from the IPG 100.
The low-amplitude AC signal received at coil 73 is amplified at a pre-amplifier 152, where it is them mixed with a 330 kHz reference signal at a mixer 154 to produce a signal with an intermediate frequency of fc-if=455 kHz. This is done in the prior art because 455 kHz comprises a well-known standard communication frequency, and as a result, receiver components are readily available to operate at this frequency. See, e.g., http://en.wikipedia.org/wiki/Intermediate_frequency. Mixer 154 can be implemented using Part No. MAX 4636, manufactured by Maxim Integrated Products, Inc.
After mixing, the up-shifted frequency is provided to a band pass filter (BPF) 156, centered at fc-if=455 kHz and with a bandwidth (BW) of 12 kHz. This BPF 156 reduces noise outside of the band of frequencies of interest (i.e., below 449 kHz and above 461 kHz), while allowing the signals from the IPG 100 (f0-if=121 k+330 k=451 kHz, and f1-if=129 k+330 k=459 kHz) to readily pass. Thereafter, the signals are passed to a limiting amplifier 158 which limits the magnitude of the signals by clipping their peaks if necessary, as is well known. Another BPF similar to BPF 156 can be provided after the limiting amplifier 158 to remove any out-of-band frequency components resulting from clipping, but this is not shown for simplicity. The BFP(s) can comprise ceramic filters, such as Part No. AHCFM2-455AL, manufactured by Toko America, Inc., or Part No. CFUM455D, manufactured by Murata Manufacturing Co.
Thereafter, the received signal is demodulated. This occurs first by sending the signals to a multiplier 160, which multiplies the signal with a phase-shifted version of the signal provided by phase shift block 162. The quad coil 163 in the phase shift block 162 is tunable to provide a 90-degree phase shift at fc-if=455 kHz, but will provide different phase shifts θ for the FSK signals of interest (f0-if=451 kHz, and f1-if=459 kHz). The output of the multiplier comprises cos(2πf)*cos(2πf+θ), or (½)cos(θ)+(½)cos(4πf+θ). A low pass filter (LPF 164) removes the AC component of this product ((½)cos(4πf+θ)), and allows only the DC component ((½)cos(θ)) to pass as analog signal 165. Because θ produced by the phase shift block 162 is different at f0-if and f1-if, the data becomes apparent at this point, although it may be substantially noisy.
The limiting amplifier 158 and multiplier 160 can comprise portions of the same demodulator integrated circuit, such as Part No. Part No. SA608DK, manufactured by NXP Semiconductors N.V.
The analog signal 165 is provided to an Analog-to-Digital converter (A/D) block 172, which can comprise a discrete block or an A/D input of a microcontroller 170 of the external controller 12 as shown. The signal 165 is sampled at an appropriate rate, and the resulting digitized values of the amplitude of the signal 165 at different points in time are stored in memory 174. Once stored, a digital filter 176, operating as software in the microcontroller 170, can operate on the stored data to remove noise and recover the data as a digital bit stream 177. The particulars of filter 176 are not important, and are not further discussed.
While the receiver and demodulation circuitry 150 of the prior art external controller 12 of FIG. 3 functions well, the inventors see room for improvement. First, circuitry 150 is relatively expensive, as it uses relatively expensive components, such as the demodulator IC and the ceramic band pass filter(s). Further, circuitry 150 has reliability and manufacturing concerns. The ceramic band pass filter(s) are fragile and can break, which is of particular concern in an external controller 12 that may from time to time be dropped by the patient. The quad coil 163 in the phase shift block 162 is also difficult to work with, as it requires special handling in manufacturing, and must be tuned by hand to ensure that it provides the proper 90-degree shift at the center frequency fc-if=455 kHz.
Given these shortcomings, the art of implantable medical devices would benefit from improved receiver and demodulation circuitry for an external controller, and this disclosure presents solutions.